As the representative of the second-generation high temperature superconducting coated conductors, YBCO (Yttrium, Barium, Copper and Oxide) which has a brittle ceramic structure need to epitaxially grow on a flexible metal tape with a biaxial texture and obtain the biaxial texture to meet the application requirements. Therefore, for the purpose of obtaining the high-performance coated superconducting wires and tapes, the key goal is to prepare the flexible metal tape which has favorable biaxial texture on surface, long length of single tape and uniformity in axial quality.
The NiW alloy is a kind of material for metal alloy tape which is selected for its good mechanical property by applying the rolling assisted biaxially textured substrate (RABiTS) method by scientific workers for years. The NiW alloy tape which has high-quality biaxial texture on surface and about 60˜100 μm thickness is produced from the NiW alloy, through heavy rolling and recrystallizing to the NiW alloy ingot (the content of W is 5˜7 at. %). But a cuboid initial NiW alloy ingot applied in a traditional rolling method is difficult to reach ideal effects in terms of the biaxial texture on surface, the length and the axial quality, upon the following reasons:
1. in order to obtain the NiW alloy tape with the high-quality biaxial texture, it needs to meet the conditions that during rolling process, the total thickness reduction of cold rolling is larger than 98%, the thickness reduction per pass is retained at certain level, and additionally, the annealing treatment cannot be performed during this process;
2. in the condition of ensuring that the thickness of the final tape is kept about 60-100 μm, in order to produce the tape which is relatively long and meet requirement 1, the initial ingot must be thickened, which will cause the increase of the total deformation of the NiW alloy ingot, resulting in serious strain hardening leading to crack sources easily generating on the both edges of the tape and tape rupture. Although the crack sources on the both edges can be removed by frequently trimming edges, the workload and the loss of materials may be increased, as well as the working hour is prolonged. More importantly, when the deformation of the ingot reaches a certain degree, its deformation effect is not remarkable anymore and a larger rolling force capable of enabling the ingot to deform needs to be provided. Therefore, more energy is consumed and loss of rolling mills is increased; even large-tonnage rolling mills are required, and accordingly, the production cost is greatly increased;
3. with regard to a cuboid initial NiW alloy ingot, hidden troubles to generate cracks described in the item 2 are brought in the subsequent cold rolling deformation processing since much stress is concentrated on the axial side edges in such processes as hot forging and hot rolling etc. before cold rolling;
4. in according to the rolling theory, the rolling process will not be smoothly finished until the width/height ratio of the NiW alloy ingot meets certain proportional relation in a rolling deformation process, that is, in case of meeting the increase in thickness (mentioned in the item 2), the ingot is need to be correspondingly widened. In other words, when the mass of raw materials is increased, the increase in width can affect the increase in length, which generates a negative influence on the expected length of the tape, and also comes up with a higher requirement on the rolling force of rolling equipment;
5. with regard to the rolling process of the long tape, the smaller the difference between the length of the final tape and the length of the initial ingot is, the more profitably the quality of the finished-product tape is well-distributed in the rolling direction, that is, the larger the contribution of the length of the initial ingot is, the longer the length of the final tape is, as well as the larger the contribution of the thickness of the initial ingot is, the more unprofitable it is.
Integrating the above reasons, in the traditional rolling method, the cuboid initial NiW alloy ingot applied in rolling can produce some unprofitable factors to producing the tape and also may cause increase of the production cost, so this is particularly remarkable for the production of the long tape. And therefore, there are some more needed to improve in selecting and rolling methods of the NiW alloy ingot.